Transparent single crystal scintillators are well known in the art for use as detectors of gamma rays, x-rays, cosmic rays, other types of high energy radiation and energetic particles of approximately 1 KeV or above. When radiation is incident on the scintillator secondary photons are generated within the crystal. These secondary photons result from the interaction of the incident radiation and an activation ion contained within the crystal. Once produced, the secondary photons can be optically coupled to a photodetector so as to produce a voltage signal that is directly related to the number and amplitude of the secondary photons. Such crystal scintillators are typically employed for medical imaging such as Positron Emission Tomography (PET), digital radiography, mineral and petroleum exploration.
An ideal detector for the detection of the above radiation and particles employs a single crystal scintillator characterised in that it exhibits:                A high density so as to provide a high stopping power on the aforesaid radiation or particles;        A high light output, this results in the production of bright visible light, typically in the blue/UV region of the electromagnetic spectrum, in response to the absorption of the aforesaid radiation or particles;        A good energy resolution, which is an important characteristic as it allows good event identification, for example in PET applications;        A short decay time, associated with the ions excited by the aforesaid radiation or particles, so as to provide detectors with a fast response time; and        A rugged structure so as to reduce the opportunity of accidental damage.        
The Prior Art teaches of various single crystal scintillator materials that have been employed in an attempt to satisfy the above criteria. One of the earliest types of scintillator employed was Thallium doped Sodium Iodide (NaI:Tl). Although capable of producing very high light outputs and being relatively inexpensive to produce NaI:Tl exhibits an inherently low density and so has a low incident radiation absorption efficiency. In addition, NaI:Tl is hygroscopic, has a slow scintillation decay time and produces a large persistent afterglow that acts to impair the counting rate performance of the material.
Table 1 provides a summary of some of the main characteristics of NaI:Tl as well as other known scintillator materials. The data within this table are taken from papers and Patents that teach of the relevant crystals, as discussed below. It should be noted that:                The light output values are relative values measured relative to the light output of NaI:Tl;        The decay times are measured in nanoseconds and refer to the time it takes for a particular activation ion of a crystal scintillator to luminesce from the excited electronic state;        The density values are measured in g/cc;        The emission peak wavelengths are measured in nanometers; and        The melting point values are measured in ° C.        
Inorganic metal oxides provide alternative single crystal scintillators devised for gamma ray detection and the like. For example a commonly employed inorganic metal oxide crystal is Bismuth Germanate (BGO). As well as being denser than NaI:Tl, BGO does not suffer from being hygroscopic. However, BGO scintillators have even slower scintillation decay times, exhibit lower light output levels that drop further with increasing temperatures and exhibit poor energy resolution values, as compared to NaI:Tl. In addition the refractive index values for BGO scintillators are relatively high so resulting in significant levels of light being lost through internal reflection processes within the crystal.
Attempts have been made to develop alternative single crystal scintillators that improve on the inherent characteristics of the aforementioned crystals. For example, Cerium activated Yttrium Orthosilicate (YSO) crystals have been developed while European Patent Application No. EP 0,231,693 teaches of a Cerium activated Gadolinium Orthosilicate (GSO) scintillator. The characteristic properties for both of these crystals are summarised in Table 1. Although exhibiting significantly faster scintillation decay times than NaI:Tl or BGO, both YSO and GSO have low densities. The light output and energy resolution values exhibited by YSO are generally good, however the inherent low density makes it a poor candidate for applications such as PET. GSO exhibits a lower light output than YSO but does have a higher density. However, the inherent poor mechanical properties of GSO make such crystals expensive to produce.
Another material that has been the subject of much development over the last few years is Cerium activated Lutetium Silicate (LSO) as taught in U.S. Pat. No. 4,958,080 and the equivalent European Patent No. 0,373,976. In particular LSO has become one of the most common crystals presently employed as a single crystal scintillator in PET as these crystals have good properties for such applications (see Table 1). LSO exhibits a fast scintillation decay time, has a fairly high density, high light output values and an average energy resolution. However, one main drawback of employing LSO as a single crystal scintillator is again the fact that it is an extremely expensive crystal to produce. This is due mainly to the fact that the melting point is very high (typically ˜2100° C.) as compared to other standard oxide crystals.
Further single crystal scintillators have been developed in attempts to improve on the working characteristics of LSO while reducing the production costs. Such attempts concentrate exclusively on introducing a substitute ion at the site of the Lutetium ions within the original LSO structure. In particular U.S. Pat. No. 6,278,832 and the equivalent European Patent Application No. EP 1,004,899 teach of mixed Lutetium Orthosilicate crystals, commonly referred to as MLS crystals. Alternatively, U.S. Pat. No. 6,323,489 teaches of a single crystal of Cerium activated Lutetium Yttrium Oxyorthosilicate (LYSO). Both MLS crystals and LYSO crystals exhibit similar physical properties to LSO but are still expensive to produce since their melting point is only slightly lower that that of LSO.
A further restricting factor that is common to LSO, LYSO and MLS crystals is the fact that they all exhibit only average levels of energy resolution, compared to GSO or NaI:Tl.
U.S. Pat. No. 5,864,141 teaches of a high resolution gamma ray imaging device that employs a Yttrium Aluminium Perovskite (YAP) crystal scintillator while U.S. Pat. No. 5,864,141 teaches of a gamma ray detector based on a Yttrium Aluminium Perovskite (YAP) crystal. A YAP single crystal scintillator is found to exhibit very fast scintillation decay times and provide very good energy resolution and light output levels. However, YAP exhibits low density levels and is again an expensive crystal to produce. The fact that YAP has superior energy resolution than LSO is due to the fact that LSO exhibits a strong non-linearity of energy response which YAP does not suffer from. The superior energy resolution has been attributed to the perovskite structure.
An alternative single crystal scintillator to YAP that is also based on the Aluminium Perovskite structure, is LuAP, which has also been known to those skilled in the art for over a decade. For example, U.S. Pat. No. 5,961,714 teaches of a method of growing Cerium activated Lutetium Aluminium Perovskite (LuAP). LuAP crystal has a significant advantage over YAP in that it exhibits a much higher density and hence a higher stopping power. This characteristic makes LuAP extremely attractive as a gamma-ray scintillator and in particular for employment within PET applications.
The main drawback with LuAP is that it is extremely difficult to manufacture due to the fact that it is metastable at high temperature, which causes decomposition of the perovskite phase at high temperature. Therefore, to date attempts to manufacture LuAP have yielded only small size samples.
Research work has also been conducted on mixed Lutetium Yttrium Aluminium Perovskite crystals e.g. Cerium activated LuYAP, which is basically a mixed crystal of LuAP and YAP. Several references, such as:                “Growth and Light Yield Performance of Dense Ce3+ doped (Lu,Y)AlO3 Solid Solution Crystals”, by Petrosyan et al, JCG 211 (2000) 252–256;        “Development of New Mixed Lu(RE3+) AP:Ce Scintillator: Comparison With Other Ce Doped or Intrinsic Scintillating Crystals”, by Cheval et al, Nuclear Inst. And methods in Phys. Res. A443 (2000) 331–341;        “Intrinsic Energy Resolution and Light Output of the Lu0.7Y0.3AP:Ce Scintillator”, by Kuntner et al, Nuclear Inst; and        Methods in Phys. Res. A 493 (2002) 131–136.describe the physical properties of a LuYAP crystal that comprises 30% Yttrium and 70% Lutetium. This LuYAP crystal requires such a high level of Yttrium in order for it not to decompose at high temperatures. However, this results in a crystal that exhibits a density and stopping power that is significantly lower than LuAP. For example in the case of LuYAP with a 30% Yttrium level the crystal density becomes comparable with LSO, namely 7.468 g/cc. The decay time of such LuYAP crystals is about 25 ns but there also exists a significant long decay time component that is detrimental to applications where a fast crystal scintillator is preferred.        